For years, electric fences have been used in agriculture settings for the purpose of containing livestock and/or repelling predators. Generally, the electric fences involve non-insulated fence wire being suspended in the air through the use of insulated members (e.g., made of plastic, porcelain, etc.) and being energized by an electric fence controller. The electric fence controller is generally provided with two output terminals. In one typical configuration, one of the terminals is connected to the electric fence wire while the other terminal is connected to ground. In turn, any animal that comes in contact with the fence wire while simultaneously being in contact with the ground receives an electric shock. Alternatively, the electric fence controller's output terminals may be connected to two fence wires positioned one above the other. In turn, any animal coming in contact with both fence wires simultaneously receives an electric shock.
In agricultural settings, the single fence wire/ground configuration and the two fence wire configuration described above involve electric fence controllers which deliver safe electric shocks. The safety parameters for such electric fence controllers are defined by safety agencies such as Underwriters Laboratories Inc. (UL). The UL document defining these safety parameters is UL69. In addition to a variety of safety requirements including acceptable construction methods, acceptable materials, and a number of other design related requirements, UL69 dictates the electrical parameters for the electric fence controller to achieve a safe output from the controller. The parameters define the maximum allowable current pulse which can be delivered to a resistive load of at least 500 Ohms. This maximum allowable current pulse or pulse segment is defined as I=20×T−0.7, where I equals the maximum current pulse amplitude in milliamps rms, and T equals the pulse-width of the current waveform in milliseconds. The UL69 safety standard also defines a required period between pulses or pulse segments. When measured at the 7 mA level, this period or “off interval” must be at least 1 second. The UL69 safety standard further defines a maximum duration for the pulse current or pulse segment, which, when measured at the 300 mA level, is 1.5 milliseconds. In summary, in addition to all the other requirements defined in the UL69 safety standard, to be considered safe, when connected to a resistive load of at least 500 ohms, an electric fence controller must have (i) an output current pulse or pulse segment (a part of a current pulse that is between any two points in time within the duration of the current pulse as per UL69) in milliamps rms less than I, where I=20×T−0.7, where T equals pulse width in milliseconds, (ii) an off-period measured at the 7 milliamp level of at least 1 second, and (iii) an on-period measured at the 300 milliamp level of not more than 1.5 milliseconds.
In recent years, electric fences have been used for law enforcement purposes. For example, electric fences have been implemented as part of perimeter fence systems. In such settings, electric fences have been used to provide a lethal electric output when contacted. Typically, the electric fence is physically located between two permanent non-electrified fences (e.g., of the chain-link type). The zone defined between the two permanent non-electrified fences typically extends across the perimeter of a prison and, quite often, is designated as a “shoot-to-kill” zone. As such, no one is allowed in the zone unless the fence is being serviced, and guards are instructed to shoot at anyone entering the zone in an effort to escape the prison. In essence, the lethal electric fence positioned between the two non-electrified fences functions as a further mechanism to dissuade prisoners from escaping via the fenced-in perimeter.
The lethal electric fence, like the non-lethal electric fence used in agricultural applications, uses a non-insulated fence wire typically supported in the air by insulated members. However, in the lethal application, the fence is constructed with a plurality of wires (e.g., typically at least twenty wires). Additionally, the fence wires are energized by a lethal electric fence energizer so that, as described above, when two wires are touched, a lethal shock is delivered. Existing lethal fence energizers generally are powered by an AC source, deliver an AC sinusoidal waveform to the fence wire, and include a linear step-up transformer. In some known systems, the lethal energizer may be used in pairs, where each energizer delivers a substantial AC voltage (e.g., several thousand volts AC), and where the energizers are of opposite polarity such that the voltage delivered to the fence wires can be effectively doubled. For example, with an output voltage of 6600 VAC at 500 mA, the load required to consume this amount of power would generally be equal to a 13,200 Ohm 3300 Watt resistor.
As described above, UL69 references a 500 Ohm resistor. This 500 Ohm resistor is generally used to represent a child that may touch the fence with the current path being from hand to foot. Likewise, an adult can be represented by a 1000 Ohm resistor. Placing a 1000 Ohm or 500 Ohm resistor across the output of a linear step-up transformer in known lethal electric fencing systems, e.g., involving lethal energizers having linear step-up transformers designed to deliver 500 mA into a 13200 Ohm resistor as exemplified above, will result in the transformer being loaded down such that the current will be higher than 500 mA, the voltage will be less than the 6600 VAC delivered into the 13200 Ohm load, and the supply current will increase compared to the condition where no load or a 13200 Ohm resistor is connected to the lethal energizer. While the current delivered to the 1000 Ohm load (or human) would be lethal, the lethal energizer is not designed for continuous operation into such a low resistance load (e.g., when an animal or human remains in contact with the lethal fence wire). The result of this situation, given enough time, is catastrophic for the lethal energizer. In turn, the fencing system would no longer provide a lethal barrier. Further, it has been found that the rate at which the lethal energizer catastrophically fails increases with the number of 1000 Ohm loads (or humans) that make contact with the fence at the same time.
To protect the lethal fence energizer and keep it from failing during false alarms, existing lethal fence systems are typically provided with a rodent wire consisting of a standard agricultural type electric fence wire placed a short height above the ground on each side of the lethal fence to keep rodents from contacting the lethal fence wires. While the rodent wire can be found to work reasonably well in protecting the lethal energizer from failing due to rodents touching the lethal fence wire, the rodent wire fails to prevent other larger bodies or conductive debris (e.g., that may be blown over one of the permanent non-electrified fences) from contacting, and ultimately, causing the lethal energizer to catastrophically fail, as described above.
Therefore, it would be advantageous to provide a lethal electric fence energizer that is not susceptible to the above limitations.